Gaseous electric discharge tubes and electrodes



P. E. GATES 0,9 GASEOUS ELECTRIC DISCHARGE TUBES AND ELECTRODES April30, 1957' Filed March 17, 1953 Rm MY 5 M m VIL U M m/ m United StatesPatent 9 GASEOUS ELECTRIC DISCHARGE TUBES AND ELECTRODES Paul E. Gates,Danvers, Mass, assignor to Sylvania Electric Products Inc., acorporation of Massachusetts Application March 17, 1953, Serial No.342,837

5 Claims. (Cl. 313-198) The present invention relates to improvedattenuator tubes and switching devices for microwave circuits. Theinvention particularly contemplates provision of a novel keep-aliveelectrode for maintaining a limited volume of a gaseous fill in gaseousdischarge devices in weakly ionized state, and to a simplified method offabricating such electrodes.

The invention is disclosed as applied to a number of gaseous dischargedevices, for example, attenuators, transmit-receive tubes, andanti-transmit-receive tubes. These devices depend for their performanceon a glow discharge in a gaseous fill between a pair of dischargeelectrodes constituting a break-down discharge gap across a waveguidetransmission path. In order to render transmitreceive tubes or duplexersmore effective, as well as to control attenuators, it is advantageous tomaintain the gaseous fill in the region of the discharge gap in a weaklyionized state. This glow discharge of a limited volume of the gaseousfill is sustained during low level signal conditions and facilitatesintense ionized discharge or breakdown across the transmission path whenhigh level energy is impressed on the waveguide. For this purpose, anadditional electrode termed a keep-alive is arranged in close proximityto the gap. In operation, a suitable potential is applied to thekeep-alive, effective to establish the desired ionization.

In order to assure that these devices shall have a relatively quickrecovery time, a gaseous fill of a mixture of an inert gas (i. e. anoble gas such as argon, whose ions combine only with each other afterionization) and a quenching gas (i. e. water) is introduced. Inoperation, when exposed to such gaseous atmosphere and the ionizingpotential, conventional keep-alives tend to deteriorate and limit theuseful life of tubes to several hundred hours, even at relatively lowcurrents. Often, it is necessary to increase the glow discharge, furtherincreasing the risk of progressively destroying the keepalive electrode.

My co-pending application Serial No. 312,604, filed September 30, 1952,assigned to the assignee of the present invention, discloses a novelmicrowave device incorporating a keep-alive electrode suitable formaintaining the required low level ionized discharge, yet minimizingdeterioration of the keep-alive, even where an aqueous quenching mixtureis used. As pointed out in my copending application, fabricationdifiiculties are encountered, due in part to the size limitationsimposed on the keep-alive and because of the physical geometry of theenvironment and electrode construction, as well as the fragile characterof suitable keep alive materials. Although the manufacturing methods andconstructions disclosed and claimed in this application representmaterial advances in fabricating of microwave devices of the aforesaidcharacter, difficulty has been encountered in attaining requisiteprecision and uniformity in manufacture without somewhat higher coststhan anticipated.

Accordingly, it is an object of the present invention to provide amicrowave gaseous discharge device having 2,790,923 Patented Apr. 30,1957 a novel keep-alive electrode effective to promote long tube life,and adapted to fabrication by simplified techniques. In particular, thepresent invention contemplates a novel yet simple method forincorporating brittle or fragile materials such as titania, havingdesirable keep alive properties, in a practical electrode construction.

It is a further object of the present invention to provide a keep-aliveelectrode assembly fabricated to as sure uniformity from unit to unit,despite the relatively tiny components of the assembly, and in a mannercompatible with minimum costs.

The above objects and further features and advantages of the presentinvention will become apparent from the following detailed descriptionof an illustrative embodiment and process shown in the accompanyingdrawings, wherein:

Fig. l is a flow diagram showing the processing of an insert forming apart of a novel keep-alive electrode; and

Fig. 2 is a longitudinal cross-section of an illustrativetransmit-receive tube containing a keep-alive electrode constructed inaccordance with the process of Fig. 1.

Referring now to the drawings and in particular to Fig. 2, there isshown a rectangular length of waveguide 10, having flanges 12, 12 forconnection to a microwave system. The ends of the waveguide 10 areprovided with resonant windows 14, 14' each of which includes a centerglass or ceramic body 16, 16 and a metal bezel of frame 18, 18'. One ormore opposite pairs of conical discharge electrodes 20, 22, and 20, 22afford a series of discharge gaps 26 at spaced intervals along thewaveguide 10, and the gap electrodes have metallic interconnection viathe waveguide wall. Electrode 22 is of closed deformable constructionfor critical adjustment by engagement from the exterior of the waveguide10 through the complementary body 24 fixed in electrode 22. Associatedwith discharge gap 26, defined by the opposite electrodes 20 and 22, isa laterally disposed pair of plates (not shown) of conventionalconstruction which provide an iris having a center opening for the gapelectrodes. The iris and the gap electrodes constitute a resonantaperture through which microwave energy of low level can be transmittedfrom one window to the other. The respective discharge gaps and irisesare separated from each other along the waveguide 10 by a spacingcommonly being a distance equal to an effective quarter wave length atthe center frequency of the band for which the device is designed.

Electrodes 2% are hollow in the preferred construction and each containsa keep-alive electrode 30 having an inner end 30a limited to a spacingof a few thousandths of an inch from the opening 20a in dischargeelectrode 20, both radially and end-wise. Discharge electrode 29 isaccordingly closer to the other discharge electrode 22 than keep-aliveelectrode 30 is to discharge electrode 22 and only the relatively smallinner end 30a. of the keepalive electrode 30 is exposed to the gaseousfill of the sealed waveguide 10 in the region of the discharge gap 26.The waveguide 10 is conventionally filled with an electron capture gaspreferably a noble gas such as argon, mixed with a quenching gas such asWater at a total pressure of the order of 25 millimeters of mercury.

In operation, a current is passed between the keep-alive electrode 30and the discharge electrode 20. in such transmit-receive tubes, it isnormal to apply several hundred volts between the keep-alive 3G and thegap electrodes. When the required potential is applied between thekeep-alive and the waveguide structure a small volume of ionized gas islocally established in a confined region adjacent gap 26, which issomewhat effective to attenuate low level signals transmitted along thewaveguide path, the attenuation being a function of the degree ofionization. Upon incidence of high level signal bursts the limitedvolume of ionized gas promotes an abrupt formation of intensely ionizeddischarges thereby to switch the waveguide transmission path from one inwhich low level signals are transmitted, to one in which high levelsignals are almost entirely reflected. The use of such devices in radarsystems and in single station receiver-transmitter units is wellunderstood by those well skilled in the art, and requires no elaborationhere.

When it is desired to increase the intensity of glowdischarge as inattenuator tubes, it is usual to greatly increase the ionizingkeep-alive current. As this current increases there will be aproportionate increase in current density which might normally result inthe accelerated deterioration of the keep-alive. This is of specialconcern in the presence of a gas fill containing water vapor where thewater vapor is cleaned up or removed from the mixture during life of thetube. Accordingly, it is desirable to employ a keep-alive electrodehaving an increased current carrying capacity, and one that does notenter into the clean-up action.

According to a feature of the present invention, the kcep-alivcelectrode 30 (Fig. 2) is formed with a rod 32, including an axialtapered or conical portion 3-!- and a cylindrical supporting portion ortip 36. Axially supported upon the portion 36 of the rod 32 is athinwall metallic sleeve 38 which cooperates with the rod 32 to form anaxial passage 40 therein. The thin-walled sleeve 38 is electricallyconnected and permanently fixed on the rod 32 by a weld 42 or the like.As seen in Fig. 1, the welding operation causes a slight deformation ofthe thinwalled sleeve 38 in the region of the weld 42.

Within the bore 40 there is an insert or tip 44 of a semiconductingceramic, specifically, partially reduced titanium oxide. The insert 44accurately fits the passage 40 and has an end face 44a exposed andspaced outwardly of the sleeve 38. An insulating sheath or glass head 46is sealed continuously on the outer surface of the rod 32, along thesleeve 36. and along the lateral surface 441? of the insert. The sheath46 confines the discharge between the keep-alive electrode 32 and theouter metallic structure of the hollow gap electrode 20 to the immediateregion of the opening 20a. This keep-alive electrode is hermeticallysealed by a glass button 48 to a sleeve or collar 50 that is brazedhermetically to the waveguide it). Accordingly, the rod 32' is fixedconcentrically of the cone 20 and critically fixed in cndwise spacedrelation thereto. The sleeve 50 is of a suitable glass to scaling metalalloy such as the well-known iron-nickel-cobalt aloys, commonlydesignated Kovar.

The titania insert 44 and the sleeve 33 are electrically andmechanically united to each other and to the conductive rod 32.Thin-walled sleeve 33 is heat-softened at 52 as by opposedresistance-Welding electrodes. The joint at 52, which may be contrastedto the true weld 42, is a sound mechanical and electrical unit ionbetween the insert 3-4 and the sleeve 33. in lieu of using conventionalwelding tips which apply pressure to opposite sides of the sleeve 33, itmay be desirable to employ an electronic torch which merely heat softensthe thin wall 33, thereby causing the wall to conform to the insert,although the latter may have minute irregularities and may be slightlysmaller than the passage in the sleeve 33 initially. Although thetitania insert is exceptionally fragile and brittle, this type ofelectrical construction utilizes the physical properties of the titaniafor improving the life of the tube without entailing difficult assemblyoperations, while assuring sound mechanical and electrical connection.

The "keep-alive electrode may be assembled in the following exemplarymanner:

Initially, and as seen in Fig. 1, titanium oxide insert 44 is preparedas an extruded rod which is then fired in hydrogen at approximately 1150Centigrade for approximately three minutes, so as to render itsemiconductive. and it is cut to an appropriate length. Thin wall sleeve38 is fixed on the supporting portion 36 of the rod 32 by a true weld42. Previously the rod 32 may be suitably cleaned of oxides, as forexample, by firing in a furnace having a hydrogen atmosphere. Thereupon.the titania insert 44 is fitted into the passage 40. Heatsoftening thesleeve 38 as at 52 provides a permanent reliable mechanical andelectrical joint, between the insert 44 and the sleeve 38. Thereafterthe assembly of the rod 32, the sleeve 38 and the projecting insert 44,is provided with the insulating sheath 46, suitably of glass, on all ofthe exposed surfaces of the assembly except for the end face 44 of theinsert. Any glass inadvertently covering the exposed end face 441: ofthe titanic insert may be ground away. The completion of the assembly ofthe keep-alive electrode 3t with the electrode construction 29 may beaccomplished by following the teachings of co-pending applications,Serial No. 237,258 of July 17, 1951, and Serial No. 226,483, filed May15, 1951, now Patent No. 2,740,186, both assigned to the assignee of thepresent invention.

Keep-alives constructed in substantially the aforementioned mannerexhibit all the advantages of the insert material as to minimizingdeterioration of the electrode and of the fill, yet have the advantagesof a strong. durable structure which may be manufactured without extremerequirements for close tolerances and meticulous processmg.

In broad concept, sleeve 38 might be formed integral with rod 32, bydrilling accurately into the center of the end of the rod, after whichthe insert could be bonded in place. The illustrated embodiment ispreferable for such tiny components as that used in the device of Fig.2, where the insert may be only 0.015 inch in diameter, and 0.125 inchlong, where the wall-thickness of sleeve 35 should be not much greaterthan 0.004 inch in this device.

Numerous modifications may occur to those skilled in the art, within thescope of the present invention, and in many instances certain featuresof the invention may be used without a corresponding use of otherfeatures. Accordingly, the appended claims should be broadly construed,consistent with the scope and spirit of the invention.

What is claimed is:

1. An electrical discharge device comprising an envelope containing anionizable gas, and a pair of discharge electrodes in said envelope, oneof said electrodes including a supporting rod, a sleeve axially mountedon said supporting rod and cooperating therewith to define a passage inthe end thereof, an insert of titanium oxide extending into saidpassage, and an insulating sheath entirely covering said sleeve.

2. An electrical discharge device comprising an envelope containing anionizable gas, a pair of electrodes in said envelope adapted to sustaina glow discharge there between, one of said electrodes including a bodyhaving a supporting part, a sleeve mounted on said supporting part andprojecting therefrom, an insert of semiconductor extending into andprojecting from said sleeve and including an end face, and an insulatingsheath sealed to the assembly of said body, sleeve and insert andcovering said assembly except for said end face.

3. A microwave electric discharge tube including a metal-walled envelopedefining a transmission path for microwave energy and arranged to admitmicrowave energy to said path, an electron capture gas fill in saidenvelope, opposed electrodes defining a discharge gap therebetween, anda keep-alive electrode adjacent said discharge gap for maintaining arestricted volume of said gas fill in weakly ionized condition, saidkeep-alive electrode including a metal support, a sleeve electricallyand mechanically united to said support, a semiconductive core supportedin said sleeve and having a face adjacent said discharge gap, and aninsulating sheath on said support, sleeve, and core covering thesurfaces thereof excepting said face.

4. In a microwave electrical discharge device including an envelopecontaining a mixture of a noble gas and water vapor and opposedelectrodes defining a discharge gap, the improvement consisting of akeep-alive electrode adjacent said discharge gap for maintaining arestricted volume of said noble gas in a weakly ionized state, saidkeep-alive electrode including a support having a sleeve fixed thereonaxially of said support, a tip of fired titanium oxide projecting fromsaid sleeve and electrically and mechanically united thereto, said tiphaving an end face exposed contiguous to said discharge gap, and alateral film of protective insulation for said keepalive electrodeproviding an insulated seal extending from said end face and through awall of said envelope.

5. An electrical discharge device comprising an envelope containing anionizable gas, a pair of opposed discharge elements defining a dischargegap in said envelope, and an electrode in close proximity to said gapand constituting a cathode for establishing a glow discharge duringoperation, said electrode including a boredefining member having a metalwall, a core of titanium oxide fixed within said bore-defining member bydeforming said bounding wall, and an insulating sheath externally ofsaid electrode and covering said electrode except for a confined regionexposing said core adjacent said discharge gap.

References Cited in the file of this patent UNITED STATES PATENTS2,159,791 Fruth May 23, 1939 2,225,970 Brian Dec. 24, 1940 2,303,514Toepper Dec. 1, 1942 2,617,957 Scott Nov. 11, 1952

